Cancer is caused by uncontrolled and unregulated cellular proliferation. Precisely what causes a cell to become malignant and proliferate in an uncontrolled and unregulated manner has been the focus of intense research over recent decades. This research has led to the targeting of surveillance mechanisms, such as those responsible for regulating the cell cycle, with anticancer agents. For example, published patent application WO 2009/103966 (CANCER RESEARCH TECHNOLOGY LIMITED) relates to the inhibition of checkpoint kinase 1 (CHK1) kinase function, with bicyclylaryl-aryl-amine compounds, in the treatment of cancer.
The main role of the cell cycle is to enable error-free DNA replication, chromosome segregation and cytokinesis. Surveillance mechanisms, the so-called checkpoint pathways, monitor passage through mitosis at several stages. One of the best characterised is the spindle assembly checkpoint that prevents anaphase onset until the appropriate tension and attachment across kinetochores is achieved (HARDWICK KG, 1998, “The spindle checkpoint”, Trends Genet 14, 1-4). The majority of proteins involved in the checkpoint exert their functions through protein binding interactions with the involvement of only a small number of kinases (MUSACCHIO A et al, 2007, “The spindle-assembly checkpoint in space and time”, Nature Reviews, Molecular and Cell Biology, 8, 379-393). A mitotic checkpoint complex (MCC) that contains three checkpoint proteins (Mad2, BubR1/Mad3, Bub3) and the APC/C co-factor, CDC20, concentrates at the kinetochores and acts as a spindle checkpoint effector. Other core proteins required to amplify the checkpoint signal include Mad1 and the kinases Bub1, Mps1 (also known as TTK) and Aurora-B (MUSACCHIO, referenced above).
One of the first components of the spindle assembly checkpoint signal, identified by a genetic screen in budding yeast, was dubbed Mps1 (monopolar spindle 1) for the monopolar spindles produced by Mps1 mutant cells (WEISS E, 1996, “The Saccharomyces cerevisiae spindle pole body duplication gene MPS1 is part of a mitotic checkpoint”, J Cell Biol 132, 111-123), however, it still remains one of the least studied checkpoint components in higher eukaryotes. Subsequently, the Mps1 gene was shown to encode an essential dual-specificity kinase (LAUZE et al, 1995, “Yeast spindle pole body duplication gene MPS1 encodes an essential dual specificity protein kinase”, EMBO J 14, 1655-1663 and also POCH et al, 1994, “RPK1, an essential yeast protein kinase involved in the regulation of the onset of mitosis, shows homology to mammalian dual-specificity kinases”, Mol Gen Genet 243, 641-653) conserved from yeast to humans (MILLS et al, 1992, “Expression of TTK, a novel human protein kinase, is associated with cell proliferation”, J Biol Chem 267, 16000-16006). Mps1 activity peaks at the G2/M transition and is enhanced upon activation of the spindle checkpoint with nocodazole (STUCKE et al, 2002, “Human Mps1 kinase is required for the spindle assembly checkpoint but not for centrosome duplication”, EMBO J 21, 1723-1732 and also LIU et al, 2003, “Human MPS1 kinase is required for mitotic arrest induced by the loss of CENP-E from kinetochores”, Mol Biol Cell 14, 1638-1651). The autophosphorylation of Mps1 at Thr676 in the activation loop has been identified and is essential for Mps1 function (MATTISON et al, 2007, “Mps1 activation loop autophosphorylation enhances kinase activity”, J Biol Chem 282, 30553-30561).
Given the importance of Mps1 in spindle checkpoint activation, the development of Mps1 inhibitors would be an asset, not only as a tool to further investigate its cell cycle-related functions, but also as a form of anticancer treatment. The first generation inhibitors of Mps1 have been described. Cincreasin, caused chromosome mis-segregation and death in yeast cells (DORER et al, 2005, “A small-molecule inhibitor of Mps1 blocks the spindle-checkpoint response to a lack of tension on mitotic chromosomes”, Curr Biol 15, 1070-1076) and SP600125, a JNK (c-Jun amino-terminal kinase) inhibitor, also disrupts spindle checkpoint function in a JNK-independent manner via the inhibition of Mps1 (SCHMIDT et al, 2005, “Ablation of the spindle assembly checkpoint by a compound targeting Mps1”, EMBO Rep 6, 866-872). Recently, three small molecule inhibitors of Mps1 were identified (KWIATOWSKI et al, 2010, “Small-molecule kinase inhibitors provide insight into Mps1 cell cycle function”, Nat Chem Biol 6, 359-368; HEWITT et al, 2010, “Sustained Mps1 activity is required in mitosis to recruit 0-Mad2 to the Mad1-C-Mad2 core complex”, J Cell Biol 190, 25-34; and SANTAGUIDA et al, 2010, “Dissecting the role of MPS1 in chromosome biorientation and the spindle checkpoint through the small molecule inhibitor reversine”, J Cell Biol 190, 73-87). Chemical inhibition of Mps1 induced premature mitotic exit, gross aneuploidy and death to human cancer cell lines (KWIATOWSKI, above). Mps1 inhibitors AZ3146 and reversine, severely impaired recruitment of Mad1, Mad2 and CENP-E to kinetochores (HEWITT, and SANTAGUIDA, above).
Dysregulation of the mitotic checkpoint is recognised as a feature of the malignant transformation process. Mitotic checkpoint dysfunction in tumors provides an opportunity for developing a therapeutic strategy using small molecules. This is based on the proposition that pharmacologic disruption of an already compromised mitotic checkpoint may selectively sensitize tumors. This observation has led to the hypothesis that inhibition of Mps1 may be of therapeutic benefit.